The present invention relates to an air-fuel ratio feedback control system for correctively controlling the air-fuel ratio of the mixture supplied to an internal combustion engine in accordance with the oxygen concentration in the exhaust gas, or more in particular to an improvement in the control characteristic immediately after engine start before warm up of the internal combustion engine.
A conventional system is well known which, for the purpose of purification of the exhaust gas, correctively controls the air-fuel ratio of the mixture by feeding back the oxygen concentration of the exhaust gas in the exhaust system of the internal combustion engine to the intake system.
Immediately after the engine start before warm up of the internal combustion engine, the oxygen sensor is still inactive and is incapable of actuating the air-fuel feedback control. Generally, before the temperature of the engine cooling water is increased to a predetermined level, the air-fuel ratio feedback control is stopped and an open loop condition is set.
At the time of engine restart or the like, the temperature of the engine cooling water is high and the ambient temperature of the oxygen sensor is low, so that the oxygen sensor is inactive. An activity monitor circuit is provided for monitoring the active or inactive state of the oxygen sensor. When the oxygen sensor shows an inactive state for a predetermined length of time, the operation of the activity monitor circuit is required to stop the air-fuel ratio feedback control and to set an open loop condition.
Demand is high for a system in which the operating region of the air-fuel ratio feedback control is widened against the engine operating region to effect purification of the exhaust gas at higher efficiency and the air-fuel ratio of the mixture gas is correctively controlled by feeding back the oxygen concentration in the exhaust gas in the exhaust system of the internal combustion engine to the intake system without any problem.
It is possible to start the air-fuel feedback control earlier before warm up of the engine by setting a lower temperature of the cooling water for feedback start. If the temperature of the cooling water is used for determining the time of activation of the oxygen sensor, however, the shortcoming is that it is impossible to set a predetermined temperature of the cooling water for starting the feedback control when the cooling conditions of the engine are not stable. If the operating region of the feedback control is broadened without the activity monitor circuit, on the other hand, an erroneous feedback control is effected thereby to adversely affect the operating performance, exhaust gas purification and the like characteristics under the inactive state of the oxygen sensor.